Exhaust gas purification systems have been developed which collect PM in exhaust gas from a diesel engine using a DPF (Diesel Particulate Filter) such as a DPD (Diesel Particulate Defuser) that is a type of DPF to reduce an amount of PM that is discharged to the outside (for example, refer to Patent Document 1).
Examples of such an exhaust gas purification system include a continuous regeneration DPF system in which a DOC (Diesel Oxidation Catalyst) is provided upstream of a DPF.
PM collected from the exhaust gas and accumulated in the DPF causes clogging of the DPF and reduces exhaust gas purification efficiency. Therefore, when the amount of PM accumulated in the DPF reaches or exceeds a fixed amount, DPF regeneration is performed in which a temperature of the exhaust gas is raised (for example, to around 500 to 600° C.) and the PM is forcibly removed by combustion (oxidation).
An amount of PM accumulation is estimated from an output value of a differential pressure sensor that measures a difference in exhaust pressure before and after the DPF. When the output value of the differential pressure sensor exceeds a predetermined differential pressure, an ECU (Engine Control Unit) assumes that the amount of PM accumulation has exceeded a predetermined amount, and either the ECU automatically starts DPF regeneration while a vehicle is in motion (automatic regeneration) or, after a DPF warning light 35a has been turned on, a driver stops the vehicle and pushes a regeneration execution switch to start DPF regeneration (manual regeneration).
In addition, there may be cases where an amount of PM accumulation is detected based on travel distance instead of on a difference in exhaust pressure before and after the DPF. In this case, DPF regeneration is automatically or manually started as described above when the travel distance exceeds a predetermined distance.
Upon start of DPF regeneration, a fuel injector of a diesel engine is controlled and an engine-out exhaust gas temperature is raised by multistage injection. Once DOC temperature has been sufficiently raised, unburnt fuel is added to the exhaust gas, and by combustion of a fuel component thereof with the DOC, a temperature of the exhaust gas flowing out from the DOC is raised to a target regeneration temperature (for example, to around 500 to 600° C.). Accordingly, the PM collected by the DPF is forcibly removed by combustion.
In doing so, feedback control of the exhaust gas temperature is performed using a signal from a temperature sensor provided on the DPF, and an addition amount of the unburnt fuel necessary for raising the temperature of the exhaust gas to the target regeneration temperature and maintaining the exhaust gas at the target regeneration temperature is constantly adjusted by PID (proportional-integrated derivation) control.
Means for adding the unburnt fuel described above can be divided into: post-injection in which fuel is once again injected into a cylinder from a fuel injector after a power stroke and unburnt fuel is added to exhaust gas remaining in the cylinder; and exhaust pipe injection in which unburnt fuel is added from an exhaust pipe injector provided at an exhaust pipe between an engine and a DPF to exhaust gas discharged from the engine and flowing through an exhaust pipe.
While post-injection enables a conventional diesel engine and a conventional exhaust gas purification system to be utilized as-is, since the unburnt fuel is directly injected into a cylinder, a problem of oil dilution occurs in that engine oil is contaminated and diluted by a fuel component and an ensuing decline in a lubricating function causes scorching of a cylinder and the like.
On the other hand, with DPF regeneration by exhaust pipe injection, the oil dilution described above no longer becomes a concern and DPF regeneration can be performed according to an amount of PM collected by a DPF.